The present invention relates to an interference cancellation apparatus and an interference cancellation method used in a communication field.
Recently, a Code Division Multiple Access (CDMA) system has been applied in a mobile communication field. In a communication using this CDMA system (CDMA communication), the system capacity is limited by mutual interference between a plurality of user signals and noise. Therefore, it is desired that the mutual interference be cancelled to improve the system capacity. In particular, in the CDMA communication, since the main factor of limiting the system capacity is the interference, the interference cancellation is specifically effective.
FIG. 1 is a block diagram illustrating a schematic configuration of a conventional base station apparatus in the CDMA communication. FIG. 1 illustrates the configuration in the case where the number of users is 3. A received signal received at antenna 1 is subjected to frequency conversion to be a baseband signal and further to sampling in radio section 2. The sampled result is demodulated according to the order which is determined in ranking determining section 3. First, a signal of user #1 of which the ranking is the highest is demodulated. It is assumed herein that the order of decreasing ranking is user #1, user #2 and user #3.
Ranking determining section 3 provides a spreading code for a signal of user #1 to correlation section 4. Correlation section 4 executes correlation calculation of the signal sampled in radio section 2 with the provided spreading code of the user #1 signal. According to the aforementioned processing, it is possible to suppress a signal component of user #2 and a signal component of user #3. The result of correlation calculation is provided to channel estimation section 10. Channel estimation section 10 executes channel estimation using the result of correlation calculation. The result of channel estimation is provided to RAKE receiving section 7. The result of correlation calculation is also provided to RAKE receiving section 7.
RAKE receiving section 7 executes RAKE combining using the correlation result in correlation section 4 and the result of channel estimation in channel estimation section 10 to obtain received data of user #1. The received data of user #1 is outputted to replica generating section 26 and subjected to convolutional calculation in replica generating section 26. Thus, a replica is generated, and a signal component of user #1 is reproduced. When the data of user #1 and the estimated result in channel estimation section 10 are correct, it is possible to completely reproduce the signal component of user #1 in the received signal.
The replica generated in replica generating section 26 is provided to subtracting section 14. The subtracting section 14 calculates a difference between the replica and an output from radio section 2. It is thereby possible to cancel adverse effects of user #1 signal on user #2 and user #3. Actually, errors may remain due to a channel estimation error and a demodulation error.
Then, ranking determining section 3 designates user #2 as a second object for demodulation, and provides information on spreading code of user #2 to correlation section 5. An output from subtracting section 14 is subjected to the same processing as in the case of user #1 in each of correlation section 5, channel estimation section 11, and RAKE receiving section 8. Thus, a received signal of user #2 is obtained, and a replica of a signal of user #2 is generated in replica generating section 13 in the same way as in user #1. Subtracting section 15 calculates a difference between the replica and the output from subtracting section 14. Interference by user #1 and user #2 are canceled in an output from subtracting section 15. However, an error may remain actually due to the channel estimation error and the demodulation error.
As a result, received data of user #3 is demodulated from the output from subtracting section 15 using correlation section 6, channel estimation section 12 and RAKE receiving section 9 in the same way as in user #1 and user #2. When the number of users is large, there is the case where interference is not adequately cancelled. In such a case, the above-described operations are carried out at a plurality of stages, and the result of a previous stage is used at a next stage, so that performance is improved.
Ranking determining section 3 executes ranking determination, for example, by determining a ranking of a next slot using outputs from channel estimation sections 10 to 12. Further, received quality estimation sections 16 to 18 respectively estimate received qualities of outputs from channel estimation sections 10 to 12 with respect to received data of respective user.
Furthermore, frame constructing sections 19 to 21 respectively construct frames of transmit data of users #1 to #3, while respectively reflecting the estimated results by received quality estimation sections 16 to 18 in transmit power control signals. Modulating sections 22 to 24 respectively modulate the transmit data. Multiplexing section 25 multiplexes modulated signals, and radio section 2 converts the multiplexed signals to high frequency signals to transmit through antenna 1. It is thus possible to perform transmit power control of a reverse link signal.
However, in the above-mentioned conventional base station, when the number of users is large, since a quality of a signal of each user is made almost constant, the quality of the signal of the highest ranking user is not remarkably high as compared to qualities of other users. Therefore, the accuracy of the generated replica is not good, and interference remains largely.
Further, the interference may be increased in some cases. In such cases, it is necessary to prepare a countermeasure such as increasing the number of stages, resulting in a problem that a calculation amount is greatly increased. Furthermore, since demodulation is executed in the order of decreasing ranking of user signal, there is another problem that processing delay is large when the number of uses is large.
An object of the present invention is to provide an interference cancellation apparatus and an interference cancellation method capable of obtaining adequate interference cancellation effects with a small amount of calculations and a small processing delay.
This object is achieved by an interference cancellation apparatus which beforehand selects a small number of users which provide large interference to the other users, such as users transmitting high rate signals, demodulates signals of selected users to generate replicas respectively, and subtracts replicas from received signals respectively, thereby canceling interference in signals of other users, and improving the system capacity.
In this case, since it is required that a signal quality of a specific user to be beforehand selected be high, the present invention provides measures, for example, for selecting a user who performs high rate data transmission. It is thereby possible to demodulate a signal of a user with high accuracy and to generate a replica with high accuracy.